Typical cables include a conductor, such as a number of copper or aluminum strands, surrounded by an insulation layer. In some instances, the life span of a cable is shortened when water enters the cable and forms micro-voids in the insulation layer. These micro-voids spread throughout the insulation layer in a tree like shape, collections of which are sometimes referred to as water trees.
Water trees are known to form in the insulation layer of electrical cables when voltage is applied to the cable in the presence of water and ions. As water trees grow, they compromise the dielectric properties of the insulation layer until failure occurs. Many large water trees initiate at the site of an imperfection or a contaminant, but contamination is not a necessary condition for water trees to propagate. In one prior art approach, water tree growth can be eliminated or retarded by removing or minimizing the water or ions, or by reducing the voltage stress.
Another prior art approach requires the injection of a dielectric enhancement restorative fluid into interstices located between the conductor strands of the cables. In a typical setup, a fluid feed tank and a vacuum tank are connected to opposite ends of the cable. The fluid feed tank consists of a fluid reservoir that is directly pressurized with compressed helium. The fluid feed tank also typically includes a sensor for determining the fluid level within the tank. For instance, an internal float valve may be disposed within the feed tank, which closes off flow when the fluid level reaches a certain minimum threshold level. The vacuum tank consists of a large reservoir that is sized to contain gases removed from the restorative fluid and the cable, thereby allowing for a complete fill of the restorative fluid within the interstices of the cable.
During operation, the fluid feed tank and the vacuum tank are left unattended, connected to the cable, for hours or even days as the injection progresses. As fluid fills the cable interstices, fluid will begin to exit the cable and fill the vacuum tank. Unless an operator interrupts or stops the injection process, fluid will continue to flow into the cable until the fluid level in the feed tank drops and the float valve is triggered to close. After a predetermined amount of time, an operator will return to the site, and he/she will remove the vacuum tank and replace the feed tank with a soak tank if supplemental fluid is needed to fully treat the cable. The soak tank would be left connected to the cable for an additional 60-90 days.
The above-described injection setup is not without its problems. For instance, if the feed tank is moved into a non-vertical position, the float valve will not be triggered to close when the fluid drops to a predetermined level. Thus, fluid from the feed tank, including compressed helium, will continue to flow into the cable. The compressed gas creates voids within the cable.
Moreover, even if the float valve properly closes, a superfluous amount of fluid will be collected in the vacuum tank before the float valve closes if the volume of fluid within the feed tank exceeds that which is needed to completely fill the cable. As the injection process takes place mostly unattended, the operator must estimate the starting fluid level within the feed tank so as to have a sufficient amount to completely fill the cable, but not so much so that the fluid simply collects in the vacuum tank.
As the injection time varies with the cable length and flow restrictions, the injection time can fluctuate greatly for each setup. Using caution and overestimating the starting level of the fluid leads to excessive amounts of waste fluid collected in the vacuum tank. If the level is too low, the cable may sit for hours or days with the valve shut off and with the vacuum connected, which can create voids in the cable as the fluid diffuses into the insulation and the vacuum pulls fluid into the vacuum tank. To help avoid this issue, operators must check each injection setup frequently.
Based on the foregoing, an improved valve assembly is needed for closing off flow within a cable injection setup assembly similar to the setup described above, which includes a cable extending between a feed tank and a vacuum tank. In particular, there is a need for an improved valve assembly that will close off fluid flow within the assembly in a reliable manner, that will help prevent the vacuum tank from withdrawing fluid from the cable when the float valve is closed, that will help prevent unnecessary waste of restorative fluid, and that will minimize the need for monitoring the setup.